1. Field of the Invention
The present invention relates to an optical head to write and read information to and from a recording medium such as an optical disc and a recording and/or reproducing apparatus provided with such an optical head.
2. Description of the Related Art
For focus servo-control of an optical head in relation to an optical disc, an objective lens is moved along the optical axis thereof towards and away from the optical disc, a laser light is irradiated through the objective lens, and a change of the laser light is returned to a focus servo signal detecting photosensor, thereby estimating whether the objective lens is in an in-focus relation to the optical disc.
When the light beam through the objective lens is nearly just focused on the optical disc, a servo mechanism is activated for an operation called "focus pull-in".
Normally the focus pull-in is within a range from dozens of .mu.m to several .mu.m from a position of the objective lens when a laser light through the objective lens is focused on a signal recording surface of the optical disc. When the distance between the objective lens and the signal recording surface of the optical disc is outside the above range, it is difficult to accurately estimate the distance between the signal recording surface of the optical disc and the objective lens and thus the focus pull-in cannot rapidly be done.
To avoid the above, it is necessary to first determine a range in which the objective lens is to be moved. The range should cover an axial deflection of the optical disc, deviation of the objective lens from a mechanical reference height, deviation of objective lens focal distance, and so forth.
The focus servo signal is detected by the astigmatic method, Foucault's method, critical angle method, or an SSD (spot size detection) method in which it is detected based on the size of light spot, and so forth whether the light beam through the objective lens is focused on the signal recording surface of the optical disc.
The focus servo pull-in range (to assure a significant signal output) can be widened with a consideration given to the optical design including optical part locations and sizes, photosensor specifications, and so forth.
However, any of the above focus servo methods can hardly allow to widen the pull-in range to more than dozens of gm in view of the servo signal gain.
Therefore, if the focus servo deviates greatly during an initial focus pull-in or due to a disturbance, the objective lens has to be moved in a wide range, to thereby detect a focus servo signal until the objective lens enters the focus servo pull-in range.
For example, if the optical disc deflects .+-.0.3 mm axially thereof, the objective lens has to be moved 0.6 mm or more.
For example, in an optical system in which the focus servo pull-in range is 10 .mu.m and the objective lens is moved over a distance WD of less than 0.6 mm, if small dust or scratch has caused the focus error signal output to be unstable and the objective lens follows up with the stable output and moves to outside the focus servo pull-in range, the objective lens will possibly collide with the optical disc at worst.
Also, if the objective lens has a large numerical aperture NA while the laser light is not changed in diameter correspondingly, the moving distance W)D of the objective lens will be short. For no collision of the objective lens with an optical disc, the optical disc should have an improved flatness, which however is very difficult to attain.
If a measuring system is available which can determine a geometric relation between the objective lens and optical disc even when the objective lens is moved only several .mu.m due to a disturbance, it is possible to provide a mechanism which prevents, based on an output signal from the measuring system, a collision of the objective lens with the optical disc. The measuring system can be used as an auxiliary focus pull-in means.
It has been proposed to use a reflective photosensor or capacitance sensor, for example, as the auxiliary focus pull-in means in order to control the position of the objective lens. However, the measuring system was problematic in that the reliability on its controlling ability is low and its size is too large.
Also it has been proposed by the Applicant of the present invention (as in the U.S. patent application Ser. No. 09/172,621 (filed on Oct. 15, 1998) to install on an actuator of the objective lens an optical system to detect a distance to an optical disc and use it for an aid to the focus pull-in operation. In this case, however, since more than one optical system is provided, a complete coordination cannot be established between them and the optical systems.
Further, in case such an auxiliary optical system is provided in the same optical path, the latter should be divided, which however will cause the amount of return light coming, by reflection, from a signal recording surface of an optical disc to the optical head to be smaller and thus the S/N ratio to be deteriorated. Also the optical path is complicated so that the optical system is large, which will limit the optical head from being designed more compactly and lead to an extra cost for the auxiliary optical system.
On the assumption that the auxiliary focus pull-in means provides a servo control signal similarly to the aforementioned normal focus servo means, if the range of servo pull-in by the auxiliary means is 0.3 mm or more, it can be considered that no collision of the objective lens with an optical disc will exist.
Since the auxiliary focus pull-in means is a protective servo against a large disturbance, it may be called "gap servo".
It may be considered that positive use of this auxiliary means contributes to speed-up of focus pull-in.